A blade-like stem generally known in the art is described in EP 0 240 815 B1. A stem corresponding generally to FIG. 6 is shown and described therein. Accordingly, that stem 1 comprises a portion comprising a prosthesis neck 7 on the one hand and a femur-anchoring portion 2 tapering towards a distal end 3 on the other hand. That femur-anchoring portion widens conically all round from the distal end 3 in the direction of the stem longitudinal axis 4. The medial narrow side 5 merges out of the cone into a continuously curved arc which ends in a plane which, running perpendicular to the prosthesis neck axis 6, terminates the prosthesis neck 7 towards the stem blade, i.e. the femur-anchoring portion 2. The prosthesis neck 7 ends in an outwardly conically tapering pin on which a spherical joint head (not shown) can be placed.
The lateral narrow side 8 widens out of the conical widened portion to form a trochanter wing 9 before merging, via a shoulder of the stem blade or anchoring portion, into the prosthesis neck termination plane.
In both cases, a stem-receiving space, that is to say a corresponding cavity, has to be provided beforehand in the femur, that being effected by means of a shaping instrument, especially a rasp, corresponding to the shape of the stem. Such shaping instruments or rasps correspond exactly to the geometry of the stem in question or differ specifically therefrom in order to obtain a predetermined undersize for a press-fit or a predetermined oversize as space for a cement mantle.
Once the hip joint has been opened and the neck of the femur resectioned, in the proximal femur the bony bearing is prepared for receiving the anchoring stem. According to the shape of the stem, the bony anchoring bed is created using a suitable shaping instrument, especially a rasp, by movement down along the stem axis. For moving down into the medullary space, which is filled with spongy bone and soft tissue, the rasp is driven forwards by means of a weight acting as a hammer or using some other suitable instrument. In the case of a curved stem axis, the shaping instrument or rasp is moved down in an arc along a curved path, while in the case of a straight stem axis the rasp is driven forward along a straight line corresponding substantially to the axis of the proximal medullary space.
For cementless anchoring of hip stems, the configuration of the anchoring portion as a straight stem has proved especially suitable clinically. This concept allows a secure implantation technique, high primary stability and good ingrowth behavior. The current surgical techniques for such stems generally require the medullary space to be opened not only in the plane of the resection surface of the neck of the femur, but also further laterally into the region of the greater trochanter. Reference is made in this respect to FIG. 4. That Figure shows that a resection of portions of the tendon insertions in that region is also generally necessary. The extent of that resection of course depends upon the individual shape of the proximal femur and upon the shaping of the straight stem, among other things.
More recently, there has been an increase in implantations of joint endoprostheses carried out using minimally invasive surgical techniques. The aim of such techniques is more rapid rehabilitation of the patient, which is associated with a reduction in pain and a shorter stay in hospital. Minimally invasive surgical techniques keep operative trauma, especially in respect of the functionally significant structures, to a very low level. For the functioning of the hip joint the important structural features are the muscles and tendons. The aim of minimally invasive implantation techniques is inter alia to avoid resections and detachments of tendon and muscle insertions in the region of the greater trochanter. Classic straight stems accordingly have disadvantages for the use of minimally invasive techniques.
To avoid resections in the region of the tendon insertions on the greater trochanter, in the case of straight stems the lateral area can be chamfered in the region of the trochanter. Straight stems having a flattened shoulder have been proposed. An example is the so-called Muller straight stem shown and described in “Technique d'implantation de protheses totales de Muller par voie laterale transgluteale”, Encyclopedie Medico-Chirurgicale (Paris) 44666, 1991.
The aim of that flattened shoulder is to avoid major defects in the region of the ridge of the trochanter. When shaping that flattened portion, a proportion of the lateral stem area, which proportion is constant within the size system, was generally configured with straight shaping inclined relative to the stem axis or with a radius. The rasp corresponding to the implant was generally made geometrically identical to the implant. From the technical standpoint, the rasp is used to create an undercut in the region of the greater trochanter, as can be seen in FIG. 5.
In the case of implantation of a hip stem, the bony bed is shaped using rasps of increasing size up to the size giving the best fit, the rasp in question following the shape of the existing bed formed by the preceding rasp size. Because the distal portion of the straight stem is affected by moving down along a straight axis, compromises are made in terms of exact fit in respect of the inclined or curved shoulder area. That is influenced, however, by the surgeon's rasp technique and individual bone quality.